Powder metallurgically produced component

ABSTRACT

The invention relates to a component produced powder metallurgically in the form of a valve guide or a valve seat ring having an iron-based matrix containing a mineral solid lubricant, with the matrix having a density ranging between 75 and 90% of the theoretical density, and with the mineral solid lubricant having a melting point that lies below the sintering temperature of the matrix and the remaining pore volume of the matrix of the component being melt infiltrated with the solid lubricant.

The invention relates to a powder metallurgically manufactured component which is exposed to tribological stresses during operation, comprising a matrix and a solid lubricant contained in the matrix. Preferably, such sintered components are valve seat rings, but also valve guiding components.

In combustion engines, the main task of a valve seat ring—in conjunction with the respective valve—is to reliably and safely seal the combustion chamber. This seal must be maintained and ensured even after many millions of load cycles. For this reason, wear in the tribological valve seat ring/valve system must be kept to a minimum.

As a rule, valve seat rings are nowadays manufactured by powder metallurgical is processes, as this allows a near net-shape production on the one hand and the well-aimed adjustment of the properties of the valve seat ring on the other through the addition of relevant components. These components are added to the powder of the base metal or base alloy that forms the matrix and bonded to it by way of a sintering process. With a view to increasing the wear resistance, hard phases (intermetallic or ceramic phases) are added, for example. However, the addition of hard phases of this nature may result in faster valve wear due to higher abrasive stresses that arise. Therefore, to reduce the total wear of the valve seat ring/valve system, solid lubricants such as MnS, MoS₂, CaF₂, BN (hexagonal) or WS₂ are also added. Through sintering, a composite material consisting of a metal matrix, hard phases and solid lubricants is produced. In this process, however, it is rarely possible to add more than about 2 wt. % of solid lubricant without causing the mechanical properties to be impaired, with the result that the influence on the wear behavior of the component can thus be considered as limited only.

Same as all sintered bodies, powder metallurgically manufactured valve seat rings—as well as valve guides—have pores. It is well known practice to infiltrate these pores in the metallic sintered body with copper in order to increase strength and thermal conductivity of the material.

It is thus the objective of the present invention to further improve the wear resistance of valve seat rings produced by powder metallurgy, especially those with integrated hard phases, but also of other powder metallurgically produced sintered components for tribological systems exposed to thermal stress, as valve guiding components, for example.

This objective is achieved with powder metallurgically manufactured components of the kind first mentioned above, in which the solid lubricant has a melting point that lies within the sintering temperature of the matrix, with the pore volume of the matrix of the component being melt infiltrated with the mineral solid lubricant.

The invention is described hereinafter on the basis of valve seat rings manufactured by powder metallurgy. It is expressly stated, however, that this can be similarly accomplished with other components exposed to tribological stresses, for instance valve guides.

The powder metallurgically produced valve seat rings proposed by the present invention comprise a conventional metal matrix, e.g. a low-alloy iron-based alloy having a low carbon content. Customary aggregates can have been sintered into this matrix, for example hard phase metal powders with intermetallic or ceramic phases and/or powders based on high-speed steel as described below, copper powders to increase thermal conductivity and solid lubricants as have already been described above. This matrix comprises a pore volume available for infiltration and, for example, can be filled in the conventional way with copper and, as proposed by the invention, with the mineral solid lubricant.

The invention proposes that at least part of the solid lubricant of the inventive sintered component is infiltrated into the pores of the matrix of the component. For this purpose, it is necessary for the solid lubricant to melt so that it can penetrate into the pores. This means that at the sintering temperature of the component, the solid lubricant necessarily must have a flowable consistency. The solid lubricant must have a melting temperature equal to or lower than the sintering temperature of the component.

Preferably, the melting point of the solid lubricant ranges between >300 and 1600° C., in particular between 450 and 1300° C. A close coordination of sintering temperature and melting temperature is expedient, i.e. the solid lubricant melting point should be close to the sintering temperature of the component. In the event a solid lubricant is used the melting point of which is much lower than the sintering temperature, it may make sense to infiltrate the finished sintered component in a separate step by reheating.

As called for by the present invention, the solid lubricant is understood to be a mineral-based one, where the term mineral denotes a metal salt or metal oxide. In particular for components that come into contact with hydrocarbon-based lubricants, such salts and oxides are considered suitable which, besides having good lubricating properties, moreover exhibit a certain affinity for hydrocarbons, i.e. are thus compatible with them. This is especially the case with some compounds of alkali and alkaline earth metals, for example their oxides, fluorides and chlorides. Particularly, the fluorides and chlorides of alkali and alkaline earth metals are preferred. In this context, the mineral solid lubricants proposed by the invention are exclusively oxides, fluorides and chlorides of alkali and alkaline earth metals, in particular the eutectic mixture of CaF₂ and BaF₂, but not conventional solid lubricants such as copper, lead, manganese sulfide or molybdenum sulfide.

Expediently, the solid lubricant comprises two or more of the above mentioned salts of alkali metals and/or alkaline earth metals. Of these substances, the salts of lithium, sodium, potassium, magnesium, calcium, barium and strontium are preferred, with those of lithium, calcium and barium being particularly preferred.

According to an advantageous variant, mixtures of salts are used which are capable of constituting a eutectic. Eutectics have a defined composition and form a homogeneous flow of melt that is not disturbed by mineral constituents. Furthermore, eutectics usually have lower melting points than the pure salts on which they are based, which often have melting points higher than those considered appropriate according to the invention. The eutectic mixtures of LiF and CaF₂, CaF₂ and MgF₂ as well as CaF₂ and BaF₂ have proven to be particularly suitable, with the eutectic mixture comprising CaF₂ and BaF₂ with 38 wt. % CaF₂ being especially suitable for valve seat rings with a sintering temperature of approx. 1100° C. The eutectic mixture of LiF and CaF₂ comprising 20.6% LiF has a melting point of about 770° C., that of MgF₂ and CaF₂ comprising 50.4 wt. % of MgF₂ has a melting point of 974° C., the melting point of the CaF₂ and BaF₂ eutectic being 1050° C. The phase diagram of the CaF₂/BaF₂ system is shown in FIG. 1.

According to the invention, it is possible to infiltrate the pore skeleton of a powder metallurgical sintered body almost entirely with a high-temperature solid lubricant. For the manufacture of valve seat rings made of an iron-based alloy with a hard phase, if applicable with a sintered-in solid lubricant and if applicable with sintered-in copper powder, the infiltrated solid lubricant in particular consists of CaF₂/BaF₂ at a ratio of 38 to 62 wt. %. At conventional sintering temperatures in the range of between 1100 and 1150° C. the fluoride mixture melts completely and infiltrates the pore skeleton, where it is distributed under the influence of capillary forces.

The mineral solid lubricant used as claimed by the invention, in particular the eutectic mixture comprising of CaF₂ and BaF₂, is not integrated into the sinter mixture, but is exclusively infiltrated into the sinter body during sintering or afterwards from the melt. The mineral solid lubricant is thus located exclusively in the pores and channels of the sintered body which are accessible from the surface.

As provided by the present invention, preferred components are made of an iron-based alloy as a matrix, which may, if applicable, be mixed with one or a plurality of hard phases and/or with copper, with a density ranging between 75 and 90%, in particular between 80 and 85% of the theoretical density. The remaining pore volume is filled substantially with infiltrated solid lubricant.

The invention moreover relates to a method for the production of tribologically stressed sintered components as have been described hereinbefore. This method comprises the following steps:

-   A1 Provision of a powder mixture for the matrix of the sintered     component; -   A2 Press forming the powder mixture mentioned against A1 into a     compact; -   B1 Provision of powder for the solid lubricant; -   B2 Press forming the powder mentioned against B1 into a compact;

C1 Sintering the compact mentioned against A2 in the presence of, preferably in contact with, the compact mentioned against B2, or alternatively

-   D1 Sintering the compact mentioned against A2 in a first step at a     first temperature T1 and -   D2 Infiltrating the sintered body mentioned against D1 in the     presence of, preferably in contact with, the compact mentioned     against B2 in a second step at a temperature T2 below the     temperature T1.

In principle, the infiltration of the solid lubricant takes place in the same way as the infiltration of copper intended to increase thermal conductivity. When the sintering temperature is reached, the solid lubricant liquefies and is caused to ingress into the pore system of the compact being sintered or already sintered due to capillary action. A homogeneous distribution of the solid lubricant is created in the sintered body, with the lubricant then being available for lubrication purposes during operation under thermal stress.

For the purpose of step B1, powder of the solid lubricant should be of an average grain size of ≤100 μm, preferably ≤50 μm and in particular ≤30 μm. In this case, the contact between the individual grains will be quite sufficient to form a eutectic mixture for melting. With a view to improving the compressibility, especially if small grain sizes are involved, it is advisable to agglomerate powder before pressing takes place, for example by spray agglomeration.

Before sintering is carried out, the compact of the matrix expediently has a density of ≤7.2 g/cm³ and in particular 6.5 to 6.95 g/cm³. In the event of compacts comprising a matrix based on iron and hard phases dispersed therein, this means an adequate pore volume is available for the absorption of solid lubricant in the order of 5 to more than 15 wt. % relative to the finished sintered component.

Moreover, the invention relates to the use of compacts comprising eutectic mixtures of alkali fluorides with alkaline earth fluorides and of alkaline earth fluorides for the infiltration of sintered bodies with solid lubricants. In particular, the use of eutectic mixtures of two fluorides is preferred and especially the use of the eutectic mixture of CaF₂ and BaF₂.

With a view to demonstrating the effectiveness of fluoride infiltration as regards the improvement of the high temperature wear behavior of valve seat ring materials, tribological examinations were performed with the aid of a tribometer of type SRV4 from Optimol Instruments GmbH. For this purpose, pins of two different is materials were rubbed against a disc made of the typical valve material 1.4882 (frequency: 10 Hz, duration: 2 h, temperature: 500° C., load: 100 N, amplitude: 2 mm). Material 1 was a typical valve seat ring material which contained a sintered-in lubricant on the basis of a heavy metal sulfide and was infiltrated with copper. Material 2 had the same basis but was infiltrated with a mixture of 62 wt. % of BaF₂ and 38 wt. % of CaF₂ instead of copper. The results of the wear tests are shown in FIG. 2. As can be seen, the material infiltrated with fluoride only shows about half as much wear as the material infiltrated with copper. The chemical composition of material 2 is displayed in Table 1. In addition to the significant improvement in wear behavior, a substantial cost reduction in the manufacture of valve seat rings can be achieved according to the invention. Furthermore, an improvement of the processability can also be expected, since fluorides, e.g. CaF₂, are frequently employed as an admixture to improve the workability of materials. It goes without saying that valve seat rings and, as the case may be, other sintered components may also have a customary composition. Aside from the admixture of one or several hard phases, other additives may be added, such as solid lubricants sintered-in to improve workability, copper powder sintered-in to raise the thermal conductivity, plasticizers such as aerosil, and wax admixtures to improve compressibility prior to sintering. 

1. Component produced powder metallurgically in the form of a valve guide or a valve seat ring having an iron-based matrix containing a mineral solid lubricant, with the matrix having a density ranging between 75 and 90% of the theoretical density, characterized in that the mineral solid lubricant has a melting point that lies below the sintering temperature of the matrix and the remaining pore volume of the matrix of the component is melt infiltrated with said solid lubricant.
 2. Component according to claim 1, characterized in that the solid lubricant has a melting point in the range of between >300 and 1600° C., in particular between 450 and 1300° C.
 3. Component according to claim 1, characterized in that the solid lubricant comprises two or several salts of alkali and/or alkaline earth metals.
 4. Component according to claim 1, characterized in that the solid lubricant contains salts of Li, Na, K, Mg, Ca, Ba and/or Sr.
 5. Component according to claim 4, characterized in that the component contains at least two salts selected from the Li, Mg, Ca and Ba salts.
 6. Component according to claim 1, characterized in that the solid lubricant consists of the eutectic mixture of two salts.
 7. Component according to claim 1, characterized in that the solid lubricant consists of a eutectic mixture of CaF₂/BaF₂.
 8. Method for manufacturing a tribological component according to claim 1 involving the steps A1 Provision of the powder mixture for the matrix of the sintered component; A2 Press forming the powder mixture mentioned against A1 into a compact; B1 Provision of powder for the solid lubricant; B2 Press forming the powder mentioned against B1 into a compact; C1 Sintering the compact mentioned against A2 in the presence of, preferably in contact with, the compact mentioned against B2, or alternatively D1 Sintering the compact mentioned against A2 in a first step at a first temperature T1 and D2 Infiltrating the sintered body mentioned against D1 in the presence of, preferably in contact with, the compact mentioned against B2 in a second step at a second temperature T2.
 9. Method according to claim 8, characterized in that the powder for the compact of solid lubricant has an average grain size of ≤100 μm, and preferably of ≤30 μm.
 10. Method according to claim 8, characterized in that the powder mixture for the matrix of the sintered component is pressed to achieve a density of 7.2 g/cm³, preferably of between 6.5 and 6.95 g/cm³.
 11. Method according to claim 8, characterized in that the powder for the solid lubricant is a powder mixture of calcium fluoride and barium fluoride.
 12. Method according to claim 11, characterized in that the powder mixture for the solid lubricant consists of 38 wt. % calcium fluoride and 62 wt. % barium fluoride.
 13. Use of compacts of a powder mixture of 38 wt. % of calcium fluoride and 62 wt. % of barium fluoride for the infiltration of valve guides and valve seat rings produced by powder metallurgy. 